_wp_staticize_emoji_for_email doesn't check for defined key in $mail array.

[nerrad]

I think this was added in 4.2 along with the emoji stuff (YAY for emoji!). This function should really be checking for whether the $mail['message'] index in the $mail array exists because other wp_mail hooks may be swapping out 'message' in favor of some other thing they are doing (in which case yes they won't get the emoji's in the content). It just prevents php notices from being thrown and breaking ajax requests using wp_mail when WP_DEBUG is on. In encountered such notice breaking things in my testing.

[nerrad]

fix for possible unset index for wp_mail array.

[pento]

In 31847:

When converting emoji in email to static images, check that the message is defined, otherwise we could cause PHP warnings.

Props nerrad.

Fixes #31708.